The present invention generally relates to limited rotation motor systems, and relates in particular to systems and methods for designing and adjusting limited rotation motor systems.
Limited rotation motors generally include stepper motors and constant velocity motors. Certain stepper motors are well suited for applications requiring high speed and high duty cycle sawtooth scanning at large scan angles. For example, U.S. Pat. No. 6,275,319 discloses an optical scanning device for raster scanning applications.
Limited rotation motors for certain applications, however, require the rotor to move between two positions with a precise and constant velocity rather than by stepping and settling in a sawtooth fashion. Such applications require that the time needed to reach the constant velocity be as short as possible and that the amount of error in the achieved velocity be as small as possible. Constant velocity motors generally provide a higher torque constant and typically include a rotor and drive circuitry for causing the rotor to rotate about a central axis, as well as a position transducer, e.g., a tachometer or a position sensor, and a feedback circuit coupled to the transducer that permits the rotor to be driven by the drive circuitry responsive to an input signal and a feedback signal. For example, U.S. Pat. No. 5,424,632 discloses a conventional two-pole limited rotation motor.
A requirement of a desired limited rotation motor for certain applications is a system that is capable of changing the angular position of a load such as a mirror from angle A to angle B, with angles A and B both within the range of angular motion of the scanner, and both defined arbitrarily precisely, in an arbitrarily short time while maintaining a desired linearity of velocity within an arbitrarily small error. Both the minimum time of response of this system and the minimum velocity error are dominated by the effective bandwidth of the system. The effective bandwidth of the system, however, is governed by many factors, including the open loop gain of the system.
A limited rotation torque motor may be modeled or represented by a double-integrator model plus several flexible modes and low frequency non-linear effects. A typical closed-loop servo system for a galvanometer includes integral actions for low frequency uncertainties and a notch filter for high frequency resonant modes. System operation is chosen at the mid-frequency range where the system is well modeled by the rigid body. For a double integrator rigid body model, there is a direct relationship between the open-loop gain and the cross-over frequency on the frequency response plot. For example, an automatic tuning system for a servowriter head positioning system is disclosed in Autotuning of a servowriter head positioning system with minimum positioning error, Y. H. Huang, S. Weerasooriya and T. S. Low, J. Applied Physics, v. 79 pp. 5674–5676 (1996).
FIG. 1 shows a model of a limited rotation torque motor system 10 of the prior art. The system 10 includes a controller 12 (e.g., a position, integral, derivative or PID controller) that receives an input command 14. The controller 12 provides a control signal to a motor 16, which moves an optical element such as a mirror to provide position changes 18 responsive to the input command 14. The system also includes a position detector 20 that provides a position detection signal 22 that is also provided to the controller 12 with the input command 14. Open-loop gain (or 0 dB cross-over variations) of the system affects closed-loop system performance if the controller is not adaptive to these variations.
In the limited rotation motor actuator, the open-loop gain is determined by the torque constant of the motor, the inertia of the mirror and rotor structure, and the gain characteristics of the power amplifier. The torque constant may change with the operation temperature. For example, the magnetic material used in the motor may have temperature coefficients of between about 0.1% C and 1% C. With a temperature change of 20 C, the resulting change in torque constant may be non-negligible. The torque constant also changes with the angle of operations. There are other factors that may affect the torque constant as well as the imperfect coil winding, which causes changes in the field density (see for example, U.S. Pat. No. 5,225,770).
The change of head from one size to another size may also cause significant changes in total inertia, and consequently the open-loop gain. Adaptive filter adjustment of open-loop gain variations due to change in mirrors is more desirable when human intervention is not required at initial set up. Other factors that may contribute to open-loop gain variations are temperature dependence of the power amplifier and changes in power amplifier circuits due to aging.
Such limited rotation motors may be used, for example, in a variety of laser scanning applications, such as high speed surface metrology. Further laser processing applications include laser welding (for example high speed spot welding), surface treatment, cutting, drilling, marking, trimming, laser repair, rapid prototyping, forming microstructures, or forming dense arrays of nanostructures on various materials.
The processing speeds of such systems are typically limited by one of more of mirror speed, X-Y stage speed, material interaction and material thermal time constants, the layout of target material and regions to be processed, and software performance. Generally, in applications where one or more of mirror speed, position accuracy, and settling time are factors that limit performance, any significant improvement in scanning system open loop gain may translate into immediate throughput improvements.
There is a need, therefore, for an improved limited rotation motor system, and more particularly, there is a need for a rotor for a limited rotation motor system that provides maximum performance.